1. Field of the Invention
The present invention relates to devices for transforming voltage. In particular, the present invention pertains to an electrical device which reduces and converts an alternating current input voltage to a direct current output voltage. More particularly, the present invention is directed to a device which efficiently converts high voltage alternating current into a lower voltage direct current for use in a dual input voltage CATV settop terminal or other electronic apparatus.
2. Description of the Prior Art
Nearly all electronic circuits today require one or more sources of stable dc voltage. The most common method to perform a voltage transformation function is using a magnetic mutual coupling or transformer to transform an alternating current impressed on a primary winding and using the magnetic circuit to induce a reduced or increased alternating current on a secondary winding. The secondary voltage is then rectified to produce a dc voltage.
Another popular method is to use a switching regulator. The advantage of the switching regulator over a conventional voltage transformer is that very little power dissipation occurs. The active switching element used in a switching power supply is either completely energized exhibiting a low conducting voltage drop or completely deenergized. Both of these attributes cause little power dissipation making the power supply inherently efficient. Switching regulators can also be coupled directly from a rectified power line with no ac power transformer. Switching regulators typically are small, lightweight and efficient dc power supplies.
Switching power supplies and power supplies relying on transformers, however, have associated problems. The most notable problem in switching power supplies is the manifestation of noise both in the power supply output and that induced back into the power source due to a high switching frequency. Furthermore, as the input voltage increases, an additional burden is placed on the individual component specifications used in the switching power supply. For power supplies using conventional transformers, higher input voltage levels typically require a more robust transformer construction yielding heavier weight, more steel and additional windings to perform the voltage transformation.
To use either of these methods of power conversion in an electrical apparatus that is manufactured to meet the demands of a worldwide market, the power supply must have an adjustable input since the input voltage level may vary in dependance upon a particular country where the device is sold. In the United States, the ac mains power supply is 60 cycles, 120 Vac. However, in other countries the frequency may vary to 50 cycles and the input voltage level may be as high as 240 Vac. For a particular manufacturer to design and build an electronic device to meet these demands, a compromise must be made in terms of expense and performance to meet the dual voltage input requirement. To design a dual input voltage power supply with either of the above methods leads to undesirable limitations.
It is therefore desirable to find a new power conversion technique for electrical devices that obviates a heavy and expensive transformer while ameliorating the negative attributes of a switching power supply. Accordingly, there is a need for a voltage device which is inexpensive, highly efficient, and manifests little undesirable interference while performing voltage conversion.